The present invention relates to the field of rollers used for transporting, guiding, or shaping industrial products and intended to be subjected to large temperatures and temperature gradients. More particularly, but not exclusively, the invention relates to rollers for very high-temperature annealing lines such as those used for fabricating very high performance steel, and in which temperatures higher than 1000° C. are reached.
Since sheet steel has very little strength at such temperatures, it is not conceivable to apply traction to the strip, so it becomes necessary to have very close support for the strip in order to prevent it from suffering creep and in order to guide it. Consequently, a very high-temperature annealing line needs to have a roller once every 0.5 meters (m) to 2 m. Each of the rollers is also motor-driven, and the set of rollers is synchronized so as to accompany the movement of the strip without applying any traction force and while minimizing friction.
The rollers used in that type of annealing line typically have diameters, by way of example and not exclusively, of about 100 millimeters (mm) and generally less than 500 mm, and a support surface of length generally lying in the range 500 mm to 3000 mm.
The rollers used in that type of industry are generally made of refractory steel with surface coatings (of the ceramic oxide, zirconia, silica, etc. type), however at temperatures above 1000° C. they present limited lifetimes in annealing lines and they need to be replaced frequently (generally once every one to five months) because of wear.
Rollers made of ceramic or of graphite that accept higher temperatures are in widespread use. However, such rollers are relatively fragile, which limits their lifetime. The invention also relates to the rollers present in lines for annealing sheet steels that are treated at lower temperatures, typically in the range 600° C. to 900° C., but that are subjected to high levels of traction. For that type of treatment, steel rollers are commonly used, however because of their significant coefficient of expansion, they can deform under the effect of temperature, which can sometimes lead to folds forming in the metal sheet (commonly referred to as “heat buckles”) or to the sheet being poorly guided (diverted). Under such circumstances, those rollers generally present larger diameters, typically in the range 500 mm to 1000 mm, with the load carrying surface possibly reaching 2000 mm in length.
Document U.S. Pat. No. 6,709,372 discloses an annealing roller for transporting a metal strip in a continuous annealing plant, the roller having a collar or shell that is made either of carbon-carbon (C—C) composite material, i.e. a material comprising carbon fiber reinforcement densified by a carbon matrix, or else of SiC—SiC composite material, i.e. a material made of SiC fiber reinforcement densified by an SiC matrix. Document U.S. 2009/036283 also describes a roller for use in high-temperature steel-making or metallurgical plant with a collar made of C—C composite material.
Although a roller having a C—C or SiC—SiC composite material ferule possesses better thermomechanical performance than a roller having a steel collar, the use of those two composite materials nevertheless presents drawbacks.
Such rollers generally require the use of a metal shaft that passes through them in order to provide mechanical strength. Under such circumstances, it is necessary to have means for providing rotary coupling between the metal shaft and the collar of composite material in order to enable the shaft to drive the collar in rotation.
However, because of the differential thermal expansion between the shaft and the collar, specific means need to be provided either to limit expansion of the shaft or to compensate for the differential expansion.
When limiting expansion, there exist devices that enable the shaft to be cooled actively against very high temperatures, e.g. by circulating a cooling fluid inside the shaft. The need to have active cooling makes the plant more complex and consumes a large amount of energy.
When compensating differential expansion, solutions have been developed to enable the metal shaft to expand while limiting the stresses it applies to the composite collar which, in contrast, expands very little at high temperature. Document U.S. 2009/036283 discloses a roller having a metal shaft with a shell or collar made of thermostructural composite material arranged thereabout. In order to compensate for differential expansion between the shaft and the shell, radial clearance is left between those two elements. Although that solution is effective in compensating differential radial expansion, it nevertheless requires grooves to be formed in the outer surface of the shaft and teeth on the inner surface of the shell for engaging in the grooves in the shaft so as to enable the shell to be driven in rotation by the shaft.
Those special shapes make fabricating the roller more complex and they increase its cost. Furthermore, positioning (centering) the collar on the shaft, when cold, also presents difficulties because of the large amount of radial clearance that is present between the shell and the shaft when cold.